Self-assembly of nanostructures

ABSTRACT

Structures and methods that include selective electrostatic placement based on a dipole-to-dipole interaction of electron-rich carbon nanotubes onto an electron-deficient pre-patterned surface. The structure includes a substrate with a first surface having a first isoelectric point and at least one additional surface having a second isoelectric point. A self-assembled monolayer is selectively formed on the first surface and includes an electron deficient compound including a deprotonated pendant hydroxamic acid or a pendant phosphonic acid group or a pendant catechol group bound to the first surface. An organic solvent can be used to deposit the electron rich carbon nanotubes on the self-assembled monolayer.

DOMESTIC PRIORITY

This Application is a CONTINUATION of U.S. patent application Ser. No.15/298,620, filed Oct. 20, 2016, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates to structures and methods of placingcarbon nanotubes (CNTs) on a substrate. More particularly, the presentinvention is generally directed to structures and methods that includeselective electrostatic placement based on a dipole-to-dipoleinteraction of electron-rich carbon nanotubes onto an electron-deficientpre-patterned surface.

CNTs can be semiconducting and therefore are of interest for electronicapplications. Semiconducting CNTs can replace or complement traditionalsemiconductors in both high-performance and low-cost thin filmtransistor devices. To make any electronic device, one needs toprecisely place the nanostructure over large areas. Various approacheshave been developed for placing CNTs on a substrate.

SUMMARY

In one or more embodiments, a method of forming a structure havingselectively placed carbon nanotubes includes providing a substrateincluding a pattern formed of a first material. The substrate is coatedwith a solution containing an electron deficient compound including apendant hydroxamic acid group or a pendant phosphonic acid group or apendant catechol group, wherein the pendant hydroxamic acid group or thependant phosphonic acid group or the pendant catechol group is reactivewith the pattern formed of the first material to form a boundself-assembled monolayer of the electron deficient compoundcorresponding to the pattern. The substrate is coated with a solutionincluding carbon nanotubes coated with an electron rich arene, whereinthe carbon nanotubes coated with the electron rich arene form adipole-to-dipole interaction with the electron deficient compound.

In one or more embodiments, a structure having a carbon nanotube (CNT)layer includes a substrate including a first surface having a firstisoelectric point and at least one additional surface having a secondisoelectric point. A self-assembled monolayer is on the first surfaceand includes an electron deficient compound including a deprotonatedpendant hydroxamic acid or a pendant phosphonic acid group or a pendantcatechol group bound to the first surface. Electron rich coated CNTs areon the self-assembled monolayer.

In one or more embodiments, a precursor for making a self-assembledmonolayer includes an electron deficient compound including a firstfunctional group selective to anchor the monolayer to a surface, whereinthe first functional group includes a phosphonic acid or a hydroxamicacid or a catechol, wherein the surface has an isoelectric point greaterthan a pKa of the phosphonic acid or the hydroxamic acid or thecatechol.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the description solely thereto, will best beappreciated in conjunction with the accompanying drawings, wherein likereference numerals denote like elements and parts, in which:

FIG. 1 depicts a perspective view of a lithographically patternedsubstrate including two different surfaces, each surface having adifferent isoelectric point in accordance with one or more embodimentsof the invention.

FIG. 2 depicts a perspective view of the lithographically patternedsubstrate of FIG. 1 subsequent to formation of a self-assembledmonolayer of an electron deficient compound so as to form an electrondeficient surface on a selected one of the two different surfaces;

FIG. 3 depicts a perspective view of the substrate of FIG. 2 includingthe self-assembled monolayer subsequent to selective electrostaticplacement of electron-rich carbon nanotubes on the self-assembledmonolayer based on a dipole-to-dipole interaction with theelectron-deficient surface; and

FIG. 4 depicts a scanning electron micrograph of an electronic structureincluding selectively placed carbon nanotubes in accordance with one ormore embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising”, “includes”, “including”, “has”,“having”, “contains” or “containing”, or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, an articleor apparatus that comprises a list of elements is not necessarilylimited to only those elements but can include other elements notexpressly listed or inherent to such article or apparatus.

As used herein, the articles “a” and “an” preceding an element orcomponent are intended to be nonrestrictive regarding the number ofinstances (i.e. occurrences) of the element or component. Therefore, “a”or “an” should be read to include one or at least one, and the singularword form of the element or component also includes the plural unlessthe number is obviously meant to be singular.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

Detailed embodiments of the structures of the present invention aredescribed herein. However, it is to be understood that the embodimentsdescribed herein are merely illustrative of the structures that can beembodied in various forms. In addition, each of the examples given inconnection with the various embodiments of the invention is intended tobe illustrative, and not restrictive. Further, the figures are notnecessarily to scale, some features can be exaggerated to show detailsof particular components. Therefore, specific structural and functionaldetails described herein are not to be interpreted as limiting, butmerely as a representative basis for teaching one skilled in the art tovariously employ the methods and structures of the present description.For the purposes of the description hereinafter, the terms “upper”,“lower”, “top”, “bottom”, “left,” and “right,” and derivatives thereofshall relate to the described structures, as they are oriented in thedrawing figures. The same numbers in the various figures can refer tothe same structural component or part thereof.

One approach to placing CNTs on a substrate involves directed assemblyof CNTs from a suspension. In this approach, a substrate is patterned todefine areas to which the CNT will have an affinity. The affinity is dueto functionalization of either the substrate or the CNT to promotecovalent or ionic bonding between the substrate and the CNT.

In another approach to place CNTs on a substrate, the prior art stamps asubstrate with an organic compound to create a substrate havinghydrophilic and hydrophobic regions. The hydrophilic region is theoriginal substrate surface and the hydrophobic region is the areastamped with the organic compound. The substrate is immersed in asolution of CNTs and dried to leave CNTs on the hydrophilic regions.However, the CNTs on the surface of the substrate are bundled (i.e. agroup of CNTs twisted together in a rope-like fashion) and/ormultilayered. Bundled or multilayered CNTs are undesirable because atransistor made from them requires higher voltage to turn the transistoron and off. The described method has another drawback in that a solutionof CNTs is not able to reach recessed hydrophilic areas having smallwidths (around or less than 200 nm). As a result, CNTs will be placed inlarge hydrophilic areas while small hydrophilic features remainuncovered. Accordingly, a CNT placement method based upon hydrogenbonding can result in poor selectivity.

Additionally, previous work on self-assembly of nanostructures utilizesexternal forces such as an electric field or electrostatic forces wherethe nanostructures were dissolved in water. These types of methods areincompatible with organic solvents.

Described herein are structures and methods for selective electrostaticplacement based on a dipole-to-dipole interaction of electron-richcarbon nanotubes and an electron-deficient pre-patterned surface. Whenthe electron-rich carbon nanotubes are deposited onto anelectron-deficient pre-patterned surface, the electron rich CNTsself-align to the electron-deficient pre-patterned surface because ofthe polarity differences, wherein the electron rich CNTs are attractedto and selectively interact with the electron deficient pre-patternedsurface. Advantageously, the method and resulting structure can be usedto selectively place the CNTs into very narrow trenches of less than 100nanometers wide and can be applied to other nanoparticles as well.

The electron deficient pre-patterned surface can generally be formed bycreating two types of surfaces with different isoelectric points usingstandard lithography and etching processes followed by selectivelyfunctionalizing an electron deficient compound on one of the surfaces.As used herein, the term “isoelectric point” generally refers to the pHat which a particular molecule carries no net electrical charge. In thepresent invention, the selected surface having a higher isoelectricpoint than the other surface is used to form the electron deficientpre-patterned surface. Typically, the isoelectric point differentialbetween the surface utilized to form the electron deficientpre-patterned surface and the other surface is at least 3 in one or moreembodiments, at least 4 in other embodiments and at least 5 in stillother embodiment. By way of example, hafnium oxide having an isoelectricpoint of about 7 can be used to form the electron deficientpre-patterned surface whereas silicon dioxide having an isoelectricpoint of about 2 can be used for the other surface, which results in anisoelectric point differential of about 5.

Suitable materials defining the surface for forming the electrondeficient pre-patterned surface include, but are not limited to, a metaloxide or a metal nitride whereas the other surface (i.e., thenon-electron deficient surface) can be formed of a non-metal oxide,e.g., a silicon oxide (Si_(x)O_(z)H_(z)) or a metal such as, but notlimited to, gold, palladium, copper, platinum, and the like. The metaloxide includes at least one metal from group IVB, VB, VIB, VIIB, VIII orIIA (CAS version) of the Periodic Table of the Elements. Illustratively,the metal oxide can be an aluminum oxide (Al₂O₃), a hafnium oxide(HfO₂), a titanium oxide (TiO_(x)), or a zinc oxide (ZnO). Exemplarymetal nitrides include silicon nitride and titanium nitride.

It is believed that surfaces with an isoelectric point greater than thepKa of the acid (e.g., hydroxamic, phosphonic, catechol) used for theself-assembly will provide directed self-assembly due to deprotonationof the acid. This is true for silicon nitride, titanium nitride aluminumoxide, hafnium oxide, and the like. Conversely, surfaces with anisoelectric point less than the pKa of the acid (hydroxamic, phosphonic,catechol) used for the self-assembly will give worse/no directedself-assembly. This is true for silicon dioxide.

By carefully selecting the isoelectric charge of one surface relative toanother, a selected one of the surfaces can be made to selectively bindthe electron deficient compound to the surface. For example, hydroxamicacids and phosphonic acids are known to selectively bind to surfacesthat are relatively basic but do not bind to surfaces which are moreacidic. Self-assembly will be provided due to deprotonation of the acidbecause the isoelectric point of the surface can be formed of a materialthat can be selected to be greater than the pKa of the acid. It isbelieved catechol will bind in a similar manner via deprotonation of thephenolic hydrogen. Conversely, surfaces with an isoelectric point lessthan the pKa of the acid used for the self-assembly will give worse/nodirected self-assembly. This is true for silicon dioxide. See, forexample, J. P. Folkers et al., “Self-assembled monolayers of long-chainhydroxamic acids on the native oxides of metals,” Langmuir, 11, 813-824(March 1995), and H. Park et al., “High-density integration of carbonnanotubes via chemical self-assembly,” Nature Nanotech., 7, 787-791(October 2012). The entire contents of each of the foregoing referencesare incorporated by reference herein. See also, U.S. Patent ApplicationPublication Number 2013/0082233, which describes an exemplary processfor decorating a surface of a substrate with a charge. In one exemplaryembodiment, the monolayer includes a positively charged pyridinium saltbearing a hydroxamic acid moiety, NMPI(4-(N-hydroxycarboxamido)-1-methylpyridinium iodide).

In view of the foregoing, the electron deficient pre-patterned surfacecan be formed by selectively reacting an electron deficient compoundincluding a terminal hydroxamic acid or phosphonic acid or catecholfunctionality with a surface having an isoelectric point greater thanthe pKa of the particular terminal acid group. Exemplary electrondeficient compounds are of formula (I) shown below.

wherein R is an electron withdrawing group, n is an integer from 1 to12, and X is hydroxamic acid, phosphonic acid or a catechol group.Exemplary electron withdrawing groups include, without limitation,fluorine groups, nitro groups, cyano groups combinations thereof, or thelike. The electron deficient compound including the terminal hydroxamicacid or phosphonic acid or catechol functionality can be solvent coatedonto the substrate. Exemplary compounds includepentafluorophenoxyhydroxamic acid or pentafluorophenoxyphosphonic acid,wherein the acid moiety selectively binds the electron deficientcompound to the patterned metal oxides present on the substrate surface.

Thus, in general, according to the present techniques, those surfacesfor which self-assembly of the monolayer is desired will be formed froma material having an isoelectric point greater than the pKa of the acid(hydroxamic, phosphonic, catechol) used for the self-assembly, and thosesurfaces for which self-assembly of the monolayer is not desired will beformed from a material having an isoelectric point that is less than thepKa of the acid. Non-limiting examples include hafnium oxide and silicondioxide, respectively.

The nanostructures can be CNTs coated with electron rich arenes, whichrender the CNTs soluble in organic solvent. The CNTs of the presentinvention can be formed by any one of several processes including, forexample, arc discharge, laser ablation, high pressure carbon monoxide(HiPCO), and chemical vapor deposition (CVD) (e.g., plasma enhancedCVD).

For example, using CVD, a metal catalyst layer of metal catalyst (e.g.,including nickel, cobalt, iron, or a combination thereof), is formed ona substrate (e.g., silicon). The metal nanoparticles can be mixed with acatalyst support (e.g., MgO, Al₂O₃, etc) to increase the specificsurface area for higher yield of the catalytic reaction of the carbonfeedstock with the metal particles. The diameters of the nanotubes thatare to be grown can be controlled by controlling the size of the metalparticles, such as by patterned (or masked) deposition of the metal,annealing, or by plasma etching of a metal layer.

The substrate including the metal catalyst layer can be heated toapproximately 700° C. The growth of the CNTs can then be initiated atthe site of the metal catalyst by introducing at least two gases intothe reactor: a process gas (e.g., ammonia, nitrogen, hydrogen or amixture of these) and a carbon-containing gas (e.g., acetylene,ethylene, ethanol, methane or a mixture of these)

A plasma can be also be used to enhance the growth process (plasmaenhanced chemical vapor deposition), in which case the nanotube growthcan follow the direction of the plasma's electric field. By properlyadjusting the geometry of the reactor it is possible to synthesizealigned carbon nanotubes.

Generally, the CNTs of the present invention can be electrically andthermally conductive, and have an essentially uniform diameter that isin a range from 1 micron (μm) to 3 μm and a length that is in a rangefrom 1 μm to 10 μm. The CNTs can also be single-walled nanotubes (SWNTs)or multi-walled nanotubes (MWNTs) (e.g., double-walled nanotubes(DWNTs)). The CNTs can also have a zigzag, an armchair, or a chiralarrangement, so long as the resulting CNT-pentacene composite shouldexhibit good charge carrier mobility (e.g., in the range of 1 cm²/V·secto 1000 cm²/V·sec).

The CNTs can also be purified (e.g., by washing in a sodium hypochloritesolution) to remove any contaminants.

The CNTs are then coated with electron rich arenes and dried. As usedherein, the term “arene” means an aromatic hydrocarbon, which cancontain a single ring or multiple rings or fused rings. In one or moreembodiments, the arene is selected from the group consisting ofoptionally substituted benzene, biphenylene, triphenylene, pyrene,naphthalene, anthracene and phenanthrene or mixtures thereof.

When the electron rich CNTs are coated onto the electron deficientpre-patterned surface, a dipole-dipole interaction occurs to provideselective placement of the CNTs on the substrate. Advantageously, theelectron rich CNTS can be deposited in an organic solvent such as bydrop casting, spin coating, or immersion coating.

After selectively placing the electron rich CNTs on electron deficientpre-patterned surface, the arenes can be removed from the CNT such as byannealing the device 100.

Referring now to FIGS. 1-3, a method of selectively placing the CNTsonto a substrate according to one or more embodiments of this inventionis given. In FIG. 1, a substrate 10 is first patterned.

The patterning process generally includes forming two types of surfaces12, 14 with different isoelectric points as previously discussed usingstandard lithography and etching processes. The lithographic step caninclude forming a photoresist (organic, inorganic or hybrid) atop asubstrate. In one or more embodiments, the photoresist can be formeddirectly on the upper surface of the substrate. The photoresist can beformed utilizing a deposition process such as, for example, CVD, PECVDand spin-on coating. Following formation of the photoresist, thephotoresist is exposed to a desired pattern of radiation. Next, theexposed photoresist is developed utilizing a conventional resistdevelopment process.

After the development step, an etching step can be performed to transferthe pattern from the patterned photoresist into at least the substrate.In one or more embodiments, the pattern can be first transferred into ahard mask material and then into the substrate. In such an embodiment,the patterned photoresist is typically, but not necessarily always,removed from the surface of the structure after transferring the patterninto the hard mask material utilizing a resist stripping process suchas, for example, ashing. The etching step used in forming the at leastone opening can include a dry etching process (including, for example,reactive ion etching, ion beam etching, plasma etching or laserablation), a wet chemical etching process or any combination thereof. Inone or more embodiments, reactive ion etching is used to form the atleast one opening. By way of non-limiting example, the at least oneopening can be a trench feature, via feature, combinations thereof, orthe like.

Each opening that is formed in the substrate is then filled withmaterial 14 having an isoelectric point different from the material 12as decried above defining the substrate or a patterned layer thereon.Alternatively, the substrate or the patterned material can be formed ofmaterial 14 and each opening filled with material 12. By way of example,the substrate can include a layer of a silicon dioxide layer formedthereon and subjected to the patterning process, wherein the patternedfeatures are subsequently filled with hafnium oxide. As noted above,material 14 is selected to have an isoelectric point that is greaterthan the pKa of the terminal acid group for the electron deficientcompound, thereby resulting in deprotonation of the acid upon contacttherewith. In contrast, material 12 is selected to have an isoelectricpoint that is less than the pKa of the terminal acid group of theelectron deficient compound such that deprotonation does not occur.

In FIG. 2, the patterned substrate is put in contact with a solutioncontaining the electron deficient compound including a terminalhydroxamic acid or a terminal phosphonic acid group or catechol group asdescribed above. The terminal hydroxamic acid or the phosphonic acid orthe catechol groups (i.e., the phenolic hydrogen) reacts with a selectedsurface, e.g., material 14, and serves to anchor the electron deficientcompound to material 14 of the substrate as indicated by arrows 16. Moreparticularly, the hydroxamic acid or the phosphonic acid or the catecholselectively binds to material 14 present on the substrate so as to forma self-assembled monolayer on the material 14.

In FIG. 3, the substrate with the self-assembled monolayer is put incontact with a solution containing CNTs, wherein the CNTs are coatedwith an electron rich arene. In one or more embodiments, the solutioncan be an organic solvent such as but not limited to aliphatics,aromatics, and mixtures thereof such as, for example, toluene,chloroform, hexane, heptane and the like. The electron rich CNTs areelectrostatically attracted to the electron deficient self-assembledmonolayer based on a dipole-to-dipole interaction.

The substrate is then rinsed in a non-polar solvent to leave a CNT layer18 above the self-assembled monolayer 16. The rinsing step removes anyexcess CNTs to form a monolayer of CNT.

FIG. 4 is a scanning electron micrograph demonstrating selectiveplacement of CNTs onto a substrate in accordance with the presentinvention. The substrate included a surface formed of silicon dioxidethat was lithographically patterned and etched to form a repeating 150nm trench feature. The trench features were filled with hafnium oxideand coated with pentafluorophenoxyhydroxamic acid to form an electrondeficient self-assembled monolayer on the hafnium oxide. A solution ofarene coated CNTs was applied to the substrate and subsequently rinsed.The excess CNTs were removed by the rinsing to provide a selectivelyplaced layer of CNTs on the electron deficient self-assembled monolayerpattern but did not form over the silicon dioxide.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of forming a structure havingselectively placed carbon nanotubes, the method comprising: providing asubstrate with a patterned surface including first and secondisoelectric points, wherein a differential between the first and secondisoelectric points is at least 3, and wherein the first isoelectricpoint is greater than the second isoelectric point; selectively bindingan electron deficient compound to the patterned surface corresponding tothe first isoelectric point to form a self-assembled monolayer, whereinthe electron deficient compound is of the formula:

wherein R is an electron withdrawing group, n is an integer from 1 to12, and X is hydroxamic acid or phosphonic acid or catechol; and coatingthe substrate with a solution comprising carbon nanotubes coated with anelectron rich arene, wherein the carbon nanotubes coated with theelectron rich arene form a dipole-to-dipole interaction with theelectron deficient compound, and wherein the electron rich arenecomprises benzene, biphenylene, triphenylene, pyrene, naphthalene,anthracene, phenanthrene, or mixtures thereof.
 2. The method of claim 1,wherein selectively binding the electron deficient compound to thepatterned surface corresponding to the first isoelectric point to formthe self-assembled monolayer comprises coating the substrate with asolution containing an electron deficient compound comprising a pendanthydroxamic acid group or a pendant phosphonic acid group or a pendantcatechol group, wherein the pendant hydroxamic acid group or the pendantphosphonic acid group or the pendant catechol group is reactive with thepattern formed of the first material to form a bound self-assembledmonolayer of the electron deficient compound corresponding to thepattern.
 3. The method of claim 2, wherein the electron withdrawinggroup comprises a fluorine group, a nitro group, a cyano group ormixtures thereof.
 4. The method of claim 1, wherein the electrondeficient compound is pentafluorophenoxyhydroxamic acid,pentafluorophenoxyphosphonic acid or mixtures thereof.
 5. The method ofclaim 1, wherein the patterned surface corresponding to the firstisoelectric point comprises a metal oxide or metal nitride.
 6. Themethod of claim 5, wherein the metal oxide includes at least one metalfrom group IVB, VB, VIB, VIIB, VIII or IIA (CAS version) of the PeriodicTable of the Elements.
 7. The method of claim 5, wherein the metal oxidecomprises aluminum oxide (Al₂O₃), a hafnium oxide (HfO₂), a titaniumoxide (TiO_(x)), or a zinc oxide (ZnO).
 8. The method of claim 5,wherein the metal nitride comprises titanium nitride or silicon nitride.9. The method of claim 1, wherein the first isoelectric point is greaterthan a pKa of the electron deficient compound and the second isoelectricpoint is less than the pKa of the electron deficient compound.
 10. Themethod of claim 9, wherein the differential between the first and secondisoelectric points is at least four.
 11. The method of claim 1, whereinthe patterned surface including the first isoelectric point comprises ahafnium oxide surface and the patterned surface including secondisoelectric point comprises silicon dioxide.
 12. The method of claim 1,wherein the electron-rich arene is selected from the group consisting ofoptionally substituted benzene, biphenylene, triphenylene, pyrene,naphthalene, anthracene and phenanthrene and mixtures thereof.
 13. Themethod of claim 1, wherein the carbon nanotubes have a length that is ina range from 1 μm to 10 μm.
 14. The method of claim 1, whereinselectively binding the electron deficient compound to the patternedsurface corresponding to the first isoelectric point to form theself-assembled monolayer comprises solvent coating the electrondeficient compound onto the patterned surface.
 15. The method of claim1, further comprising removing the electron rich arenes from the carbonnanotubes by annealing.